Advances in technology have led to the development of medical devices that can be implanted within a living organism, such as a human, to provide treatment or monitoring. For example, a medical device may detect when a seizure occurs in a patient. Early detection of a seizure may allow appropriate responsive action to be taken. Such actions may include sending an alert signal to the patient or a caregiver, initiating a treatment therapy, or taking remedial action such as making the patient and/or an environment around the patient safe. One way to detect seizures is by monitoring a heartbeat of the patient to determine whether the heartbeat increases beyond a threshold. However, other factors can also cause the heartbeat to increase beyond the threshold. Thus, seizure detection based on a heartbeat may be subject to a significant number of false positives.